Displacement sensors provide an indication of the position of an object and its movement. A wide variety of mechanical, electrical, and optical displacement sensors are known. The selection of a particular type of displacement sensor is made responsive to the geometry of the displacement, the magnitude of the displacement, whether the displacement readout is to be local or remote, and other factors.
An optical fiber that is straight or has relatively gentle bends transmits light energy with total internal reflection, so that substantially no light energy is lost as the light is propagated through the optical fiber. However, if the optical fiber is bent to a sharper curvature past a critical minimal value, the total internal reflection is disrupted, and light energy is lost responsive to the sharpness of the bend in the optical fiber. In one type of optical displacement sensor, an optical fiber is positioned so that it is bent by the displacement of an object. The greater the displacement of the object past the critical minimal value, the greater is the bending of the optical fiber and the greater is the attenuation of the light propagated through the optical fiber. The extent of the displacement of the object is determined from a calibration of the bending and the attenuation of light.
Optical fiber displacement sensors have many advantages, including light weight, remote readout, and the ability to provide the readout in either a light-based or an electrical-based readout. Existing optical fiber displacement sensors work well for a variety of applications, but are limited in their application and even ineffective in others. There is a need for an improved approach to optical fiber displacement sensors that extends the use of such sensors to applications for which they are not now suitable. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical fiber displacement sensor and a method for performing optical fiber displacement sensing. The displacement sensing approach is highly accurate. It also provides temperature compensation, so that the sensed position is not affected by the temperature of the object being sensed. The present approach is suitable for applications wherein small displacements from zero displacement are to be measured, and for applications where displacements over a range extending through an undisplaced state are to be measured. In a conventional optical fiber displacement sensor, the attenuation begins only when a critical radius for loss of total internal reflection (TIR) is reached. There is no attenuation and no output reading for small displacements. In the present approach, on the other hand, there is an output signal even with very small displacements. Additionally, the present approach senses displacement continuously from a positive displacement of the object, through zero displacement of the object (i.e., an undeformed object), and to a negative displacement of the object.
In accordance with the invention, a differential output optical fiber displacement sensor comprises a structure subject to mechanical displacement over a displacement range, and a pair of optical fibers. The pair of optical fibers includes a first optical fiber having a first-fiber input end, a first-fiber central section contacting the structure, and a first-fiber output end, and a second optical fiber having a second-fiber input end, a second-fiber central section contacting the structure, and a second-fiber output end. (There may be more than one first optical fiber and more than one second optical fiber.) The first-fiber central section and the second-fiber central section bend inversely to each other over at least part of the displacement range when the structure experiences mechanical displacement. A light source introduces a first input light signal into the first-fiber input end, and a second input light signal into the second-fiber input end. A light detector detects a first output light signal from the first-fiber output end, and a second output light signal from the second-fiber output end. A differential calculator calculates a difference value responsive to a difference between the first output light signal and the second output light signal and provides an output signal responsive to the difference value. Preferably, at least one of the first optical fiber and the second optical fiber is not totally internally reflective over all displacements in the displacement range.
The structure may comprise a mechanical movement. In one case, the mechanical movement includes a first fixed support contacted by the first optical fiber, a second fixed support contacted by the second optical fiber, and a movable element intermediate between the first fixed support and the second fixed support. The movable element contacts each of the first optical fiber and the second optical fiber over at least part of the displacement range. The movable element may be pivotably movable or linearly movable.
In such a mechanical movement, the first fixed support may comprise at least one, and preferably more than one, first fixed-support fulcrums contacted by the first optical fiber so that the first optical fiber may bend over the first fixed-support fulcrum. The second fixed support may comprise at least one, and preferably more than one, second fixed-support fulcrum contacted by the second optical fiber so that the second optical fiber may bend over the second fixed-support fulcrum. The movable element intermediate between the first fixed support and the second fixed support may comprise at least one, and preferably more than one, movable-element fulcrum disposed between the first fixed support and the second fixed support. The movable element contacts each of the first optical fiber and the second optical fiber over at least part of the displacement range to bend the respective optical fibers over their respective fixed-support fulcrums. In one design, each of the fulcrums is a support pin extending parallel to the other support pins.
The light source may include a single light-producing element producing both the first input light signal and the second input light signal. The light source may include instead a first light-producing element producing the first input light signal, and a second light-producing element producing the second input light signal. The first input light signal and the second input light signal may be of substantially the same value, or of different values.
The light detector may include a single light sensor producing both the first output light signal and the second output light signal. The light detector may instead include a first light sensor producing the first output light signal, and a separate second light sensor producing the second output light signal.
A method for performing differential output optical fiber displacement sensing comprises the steps of providing a first optical fiber and a second optical fiber, bending the first optical fiber and the second optical fiber such that a single mechanical movement causes a first bending deformation in the first optical fiber to increase and a second bending deformation in the second optical fiber to decrease, and measuring a difference value responsive to a difference between a first light intensity transmitted through the first optical fiber and a second light intensity transmitted through the second optical fiber. A temperature of operation may be changed concurrently with the steps of bending and measuring.
The present approach provides an output signal responsive to the displacement of the object, which may be a mechanical movement. Both optical fibers are sensing fibers. The output signal is therefore highly sensitive to the displacement, and yet is insensitive to temperature changes because the temperatures of both optical fibers are the same. There is an output signal for even very small displacements. The present approach senses displacement continuously from a positive displacement of the object, through zero displacement of the object (i.e., an undeformed object), and to a negative displacement of the object, without any range of displacement where there is no displacement signal. The optical fibers may be bent asymmetrically, so that the output signal is tailored to be greater in a selected direction or extent of deformation.